Targeted delivery of murine interferon (IFN)-gamma using a recombinant fowlpox virus: natural killer (NK) cell recruitment to regional lymph nodes and priming of tumor-specific host immunity. IFN-gamma is a proinflammatory cytokine that also acts as a potent immunomodulatory agent. In this study, a replication-deficient recombinant avian (fowlpox) virus was engineered to express the murine IFN-gamma gene (rF-MuIFN-gamma) with the rationale of delivering concentrated levels of the cytokine to a local tissue microenvironment. Subcutaneous (s.c.) rF-MuIFN-gamma administration resulted in IFN-gamma production that was restricted to the tissue microenvironment of the injection site and (2) was biologically active, as evidenced by a significant increase of class I major histocompatibility complex (MHC) expression levels in s.c. growing tumors following rF-MuIFN-gamma administration. Infection of a highly tumorigenic murine cell line, MC38, with rF-MuIFN-gamma functioned as an effective tumor cell-based vaccine by protecting mice from the formation of primary tumors and from subsequent tumor challenge. The cell-based vaccine was completely ineffective if mice were vaccinated with MC38 cells either pretreated with rIFN-gamma or infected with the wild-type fowlpox virus (FP-WT). Analysis of the regional lymph nodes draining the site of injection of the rF-MuIFN-gamma-based tumor cell vaccine revealed the presence of tumor-specific cell lysis (CTL) as well as a significant amount of lysis directed at NK-sensitive YAC-1 cells. Flow cytometric analyses coupled with functional assays confirmed the sustained presence of NK1.1(+) cells within those draining lymph nodes for up to 5 days after rF-MuIFN-gamma injection. Mice treated with NK cell-depleting antibodies prior to the injection of the rF-MuIFN-gamma-infected MC38 tumor cells were not protected from primary tumor growth;analysis of the lymph nodes draining the injection site in NK-depleted mice revealed an accompanying loss of the tumor-specific CTL activity. The findings provide evidence that NK cells, known for their contributions to host innate immunity, also provide immunoregulatory signals required for the development of an adaptive immune response, which, in turn, protected vaccinated mice against tumor growth. Interferon-alpha (IFN-&amp;#945;) is a pleiotropic cytokine possessing immunomodulatory properties that may improve the efficacy of therapeutic cancer vaccines. The aim of this project was to evaluate the effectiveness and compatibility of combining recombinant IFN-&amp;#945; with poxvirus vaccines targeting the human carcinoembryonic antigen (CEA) in murine models of colorectal and pancreatic adenocarcinomas, where CEA is a self-antigen. The phenotypic and functional effects of IFN-&amp;#945;were evaluated in the draining inguinal lymph nodes of tumor-free mice. We studied the effect of the site of IFN-&amp;#945; administration (local versus distal) on antigen-specific immune responses to poxvirus vaccination. Mechanistic studies were conducted to assess the efficacy of IFN-a and CEA-directed poxvirus vaccines in tumor-bearing CEA transgenic mice. We identified a dose and schedule of IFN-&amp;#945;that induced a locoregional expansion of the draining inguinal lymph nodes and improved cellular cytotoxicity (natural killer and CD8+) and antigen presentation. Suppression of the vaccinia virus was avoided by administering IFN-&amp;#945;distal to the site of vaccination. The combination of IFN-&amp;#945;and vaccine inhibited tumor growth, improved survival, and elicited CEA-specific CTL responses in mice with CEA+ adenocarcinomas. In mice with pancreatic tumors, IFN- &amp;#945;slowed tumor growth, induced CTL activity, and increased CD8+ tumor-infiltrating lymphocytes. These data suggest that IFN- &amp;#945;can be used as a biological response modifier with antigen-directed poxvirus vaccines to yield significant therapeutic antitumor immune responses. This study provides the rationale and mechanistic insights to support a clinical trial of this immunotherapeutic strategy in patients with CEA-expressing carcinomas.